The arc discharge was the first available method for the production of both multiwall nanotubes (MWNTs) and single wall nanotubes (SWNTs). This technique has been in use for a long time for the production of carbon fibers and it is probable that nanotubes were observed before 1991 but not recognized. An arc is struck between two graphite electrodes in a gas atmosphere. MWNTs produced by arc discharge are long and straight tubes closed at both ends with graphitic walls running parallel to the tube axis. Iijima et al. (“Single-Shell Carbon Nanotubes of 1-nm Diameter”, Nature 363:603–605, June 1993) and Bethune et al. (“Cobalt-Catalysed Growth of Carbon Nanotubes with Single-Atomic-Layer Walls,” Nature, 363:605–607, June 1993) reported that an arc discharge with a cathode containing metal catalysts (such as cobalt, iron or nickel) mixed to graphite powder results in a deposit containing SWNTs. SWNTs are usually assembled in ropes but some single tubes can also be found in the deposits. Another method to grow SWNTs using laser ablation was demonstrated in 1996 by Smalley's group and has prompted a lot of interest. Specifically, Thess et al. showed that the synthesis could be carried out in a horizontal flow tube under a flow of inert gas at controlled pressure [“Crystalline Ropes of Metallic Carbon Nanotubes,” Science, 273, 483–487 (1996)]. In this set-up the flow tube is heated to ˜1200° C. by a tube furnace. Laser pulses enter the tube and strike a target consisting of a mixture of graphite and a metal catalyst such as Co or Ni. SWNTs condense from the laser vaporization plume and are deposited on a collector outside the furnace zone. These methods of the first generation have some disadvantages as they require complicated purifying processes after their synthesis to make high purity carbon nanotubes, and it is also difficult to control the structure of the carbon nanotubes and to perform a vertical alignment of the tubes.
In the second-generation methods, there has been developed the gas phase combustion synthesis for the purpose of mass production of carbon nanotubes. Recently, active researches have been carried out on methods capable of the synthesis of carbon nanotubes with vertical alignment, e.g., chemical vapor deposition (“CVD”), plasma enhanced (“PE”) CVD, radio frequency (“RF”) plasma CVD, microwave PE CVD and thermal CVD. It has been possible to perform the synthesis in low temperature, high purity synthesis, and the synthesis on large area substrate as well as the vertical alignment synthesis of carbon nanotubes by second generation methods (see, Korean Patent Laid-open No. 2001-49668). However, the second-generation methods are the batch-type processes, which require vacuum, and thus, the manufacturing cost of carbon nanotubes is extremely high.
Recently, some researchers have tried to synthesize carbon nanotubes using flame instead of the second-generation methods as described above. The flame is in the form of a diffusion flame wherein the oxidizing agent surrounds the fuel. The problem which exists is that in the case of using a conventional method for combustion synthesis of carbon nanotubes, it is impossible to directly collect samples of the carbon nanotubes produced in the flame zone or to install a substrate over the flame zone wherein oxidation may not be occur.